Ison Elon A, Cessarich Jeanette E, Travia Nicholas E, Fanwick Phillip E, Abu-Omar Mahdi M
Brown Laboratory, Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, USA.
J Am Chem Soc. 2007 Feb 7;129(5):1167-78. doi: 10.1021/ja065551p.
A facile method is described for the synthesis of cationic Re(VII) cis oxo imido complexes of the form [Re(O)(NAr)(salpd)+] (salpd = N,N'-propane-1,3-diylbis(salicylideneimine)), 4, [Re(O)(NAr)(saldach)+] (saldach = N,N'-cyclohexane-1,3-diylbis(salicylideneimine)), 5, and [Re(O)(NAr)(hoz)2+] (hoz = 2-(2'-hydroxyphenyl)-2-oxazoline) (Ar = 2,4,6,-(Me)C(6)H(2); 4-(OMe)C(6)H(4); 4-(Me)C(6)H(4); 4-(CF3)C6H4; 4-MeC(6)H(4)SO(2)), 6, from the reaction of oxorhenium(V) [(L)Re(O)(Solv)+] (1-3) and aryl azides under ambient conditions. Unlike previously reported cationic Re(VII) dioxo complexes, these cationic oxo imido complexes can be obtained on a preparative scale, and an X-ray crystal structure of [Re(O)(NMes)(saldach)+], 5a, has been obtained. Despite the multiple stereoisomers that could arise from tetradentate ligation of salen ligands to rhenium, one major isomer is observed and isolated in each instant. The electronic rationalization for stereoselectivity is discussed. Investigation of the mechanism suggests that the reactions of Re(V) with aryl azides proceed through an azido adduct similar to the group 5 complexes of Bergman and Cummins. Treatment of the cationic oxo imido complexes with a reductant (PAr(3), PhSMe, or PhSH) results in oxygen atom transfer (OAT) and the formation of cationic Re(V) imido complexes. [(salpd)Re(NMes)(PPh(3))(+)] (7) and [(hoz)2Re(NAr)(PPh(3))(+)] (Ar = m-OMe phenyl) (9) have been isolated on a preparative scale and fully characterized including an X-ray single-crystal structure of 7. The kinetics of OAT, monitored by stopped-flow spectroscopy, has revealed rate saturation for substrate dependences. The different plateau values for different oxygen acceptors (Y) provide direct support for a previously suggested mechanism in which the reductant forms a prior-equilibrium adduct with the rhenium oxo (ReVII = O<--Y). The second-order rate constants of OAT, which span more than 3 orders of magnitude for a given substrate, are significantly affected by the electronics of the imido ancillary ligand with electron-withdrawing imidos being most effective. However, the rate constant for the most active oxo imido rhenium(VII) is 2 orders of magnitude slower than that observed for the known cationic dioxo Re(VII) [(hoz)2Re(O)(2)(+)].
描述了一种简便的方法，用于在环境条件下由氧铼（V）[(L)Re(O)(Solv)+]（1 - 3）与芳基叠氮化物反应合成形式为[Re(O)(NAr)(salpd)+]（salpd = N,N'-丙烷 - 1,3 - 二基双（水杨醛亚胺））、4、[Re(O)(NAr)(saldach)+]（saldach = N,N'-环己烷 - 1,3 - 二基双（水杨醛亚胺））、5以及[Re(O)(NAr)(hoz)2+]（hoz = 2 - (2'-羟基苯基)-2 - 恶唑啉）（Ar = 2,4,6 - (Me)C(6)H(2); 4 - (OMe)C(6)H(4); 4 - (Me)C(6)H(4); 4 - (CF3)C6H4; 4 - MeC(6)H(4)SO(2)）、6的阳离子铼（VII）顺式氧亚胺配合物。与先前报道的阳离子铼（VII）二氧配合物不同，这些阳离子氧亚胺配合物可以在制备规模上获得，并且已获得[Re(O)(NMes)(saldach)+]、5a的X射线晶体结构。尽管salen配体与铼的四齿配位可能产生多种立体异构体，但在每种情况下都观察到并分离出一种主要异构体。讨论了立体选择性的电子合理化。机理研究表明，Re(V)与芳基叠氮化物的反应通过类似于Bergman和Cummins的第5族配合物的叠氮加合物进行。用还原剂（PAr(3)、PhSMe或PhSH）处理阳离子氧亚胺配合物会导致氧原子转移（OAT）并形成阳离子铼（V）亚胺配合物。[(salpd)Re(NMes)(PPh(3))(+)]和[(hoz)2Re(NAr)(PPh(3))(+)]（Ar = 间甲氧基苯基）（9）已在制备规模上分离并进行了全面表征，包括7的X射线单晶结构。通过停流光谱监测OAT的动力学，揭示了底物依赖性的速率饱和。不同氧受体（Y）的不同平台值为先前提出的机理提供了直接支持，其中还原剂与铼氧（ReVII = O<--Y）形成预平衡加合物。对于给定底物，OAT的二级速率常数跨越超过3个数量级，受亚胺辅助配体的电子性质显著影响，吸电子亚胺最有效。然而，最活泼的氧亚胺铼（VII）的速率常数比已知的阳离子二氧铼（VII）[(hoz)2Re(O)(2)(+)]慢2个数量级。
